Multi-stable electronic circuits are quite common, and are used for various purposes. Certain types of bandgap reference circuits, for example, have two stable operating states, including a normal operating state in which a reference voltage is provided at an output and an off state in which the output level is zero. On power up, the output level of such a bandgap reference circuit initially starts at 0 Volts, which is the correct voltage for one of the stable operating states. The circuit may actually remain in its latched off state after being powered up.
This startup problem might not be encountered where there are large amounts of circuit noise or some sort of startup circuit to bias the bandgap reference circuit toward its normal operating state. One conventional way to start up a bandgap reference circuit is to use a pull-up resistor or transistor at its output. As the power supply rises from 0 Volts, the pull-up provides a bias current, which causes the bandgap reference circuit to favor and snap into its normal operating state.
Many bandgap reference circuits include an operational amplifier or “opamp” to generate the reference voltage. Once the power supply is at a high enough level for the opamp to function properly, the current required to pull the bandgap reference circuit out of a latched off state condition can be quite high, especially if the output stage of the opamp has a strong current sinking capability, as is typically the case.
In general, a pull-up should be able to overdrive the output of an opamp while power supply voltage is too low for the opamp to be functional. However, if a bandgap reference circuit shuts off after it was fully operational, due to noise or other conditions for instance, the pull-up might not be strong enough to overdrive the opamp output and the bandgap can remain latched off.
The above issues relating to a latched off state similarly apply to other types of multi-stable circuits, which may enter and remain in an undesired state.